1. Field of the Invention
The present invention relates generally to a printing apparatus for printing color images through superposed transfer of different color images by means of a plurality of electrostatic recording units having an electrophotographic recording print function for use in printers, copiers, etc., and more particularly to a printing apparatus for detecting and correcting relative positional offsets of color images formed by a plurality of removable electrostatic recording units.
2. Description of the Related Art
Up until now, in color printing apparatuses using electrophotographic recording, electrostatic recording units for four colors, black (K), cyan (C), magenta (M) and yellow (Y) have been arranged in tandem in the direction of conveyance of record paper. The electrostatic recording units for four colors form latent images through the optical scanning of photosensitive drums based on image data, develop these latent images with color toners from developing vessels, and thereafter transfer the yellow (Y), magenta (M), cyan (C) and black (K) toner images in the mentioned order and in a superposed manner onto the record paper being conveyed at a certain velocity, the resultant images being finally subjected to a thermal fixation through a fixing device. In case of running short of color toners, the entirety or part of the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K) must be replaced. To this end, the electrostatic recording units are provided with a structure ensuring easy mounting and dismounting with the apparatus cover opened. In the color printing apparatus comprising the electrostatic recording units for Y, M, C and K arranged in tandem in the record paper conveyance direction, on the other hand, the requirement for enhancing the quality of the color printing is to reduce positional offsets of toner images transferred by the electrostatic recording units onto the record paper in motion to thereby enhance the accuracy of color matching. Providing that the resolutions on the record paper in the horizontal scanning direction (the direction orthogonal to the conveyance direction) and in the vertical scanning direction (the direction in which the record paper is conveyed) are respectively 600 dpi, the pixel pitch results in about 42 .mu.m, and hence the positional offset must be suppressed to 42 .mu.m or below.
In the case of the conventional tandem type color printing apparatus, however, it was difficult to realize the color matching ensuring the positional offsets less than 42 .mu.m through the mechanical machining accuracy or assembling accuracy since the removable electrostatic recording units for Y, M, C and K present larger positional offsets than the fixed electrostatic recording units. In Japanese Patent Laid-open Pub. No. Hei8-85236 for instance, to solve this problem, test pattern resist marks are transferred onto the transfer belt at four points on rectangular corners and read by a CCD to detect the amount of positional offsets of detection coordinates of the resist marks relative to previously set absolute reference coordinates, so that upon the output of image data to a laser scanner, the output coordinate positions are corrected on the basis of the amount of offsets detected. However, such conventional positional offset detection and positional offset correction necessitate the detection of the positional offsets of the resist marks relative to the absolute coordinates, on all the electrostatic recording units for yellow (Y), magenta (M), cyan (C) and black (K). In addition, the use of the CCD for the detection of the resist marks leads to extended processing time for positional offset detection and to increased amount of the hardware, resulting in raised production costs. Although it is also conceivable to correct the positional offsets through the mechanical adjustability of the light emitting array, it would be difficult to realize the adjustment capable of suppressing the positional offset to 42 .mu.m or below. For this reason, the color printing apparatus using the light emitting array has often caused positional offsets as large as 300 .mu.m for instance, making it impossible to realize the satisfactory printing quality through the superposition of the color components.